DESCRIPTION (From the Applicant's Abstract): Current ideas of synapse formation suggest that muscle is patterned by signals, such as agrin, provided by motor neurons. Our recent studies, however, have revealed that muscle is pre-patterned in the absence of innervation. We found that motor axons in top 2b mutant embryos reach their targets but fail to grow or branch within limb or diaphragm muscles. To our surprise, we found that AChRs are clustered in the central region of muscle, despite the absence of motor axons within the muscle. These results suggest that the expression pattern of AChRs in skeletal muscles is determined, at least in part, by mechanisms that are autonomous to muscle and suggest that muscle is pre-patterned, independent from signals provided by motor neurons. The experiments described in this proposal are designed to determine how pre-patterning of muscle is established, whether muscle pre-patterning might regulate where axons terminate and form synapses and how innervation might regulate muscle pre-patterning. We will determine (1) whether motor innervation requires neural or muscle expression of top 2b, (2) whether signals from motor axons are required to pre-pattern AChRs in skeletal muscle, (3) whether MuSK or agrin are required to establish muscle pre-patterning, (4) whether additional skeletal muscle proteins are pre-patterned in muscle, (5) whether genes encoding synaptic proteins are pre-patterned in muscle, (6) whether pre-patterned molecules might have a role in specifying the site of motor innervation, (7) whether motor axons or electrical activity suppress muscle pre-patterning, and (8) whether a distinct myoblast lineage might be a source of the pre-pattern.